The present invention relates to a spectrometry measuring apparatus of a spectrograph type using a concave holographic diffraction grating manufactured by a holographic method.
A conventional spectrometry measuring apparatus of a spectrographic type using a concave holographic diffraction grating has been disclosed in Japanese Patent of Publication No. 62-27681 and Japanese Patent of Publication No. 63-46371.
For the conventional apparatus, an exposure layout for a concave holographic diffraction grating is shown in FIG. 8, and a spectral layout is shown in FIG. 9. As shown in FIG. 8, concave diffraction grating 23 is exposed by two exposure light sources 21, 22 near Rowland circle 20. Concave diffraction grating 23 exposed in the exposure condition in FIG. 8 disperses and converges a light to be measured diverged from incident slit 24 installed as shown in FIG. 9. An array sensor is placed at spectrograph focal position 25, and the spectral intensity of the dispersed light is measured.
In the concave holographic diffraction grating, since the diffraction grating also serves as a condenser, as compared with the ordinary plane diffraction grating, a collimating mirror or a condensing mirror is not necessary, and hence, the measuring apparatus is compact. It is a further advantage of this diffraction grating that the aberration state of the spectral graph in the condenser is excellent because an aberration can be corrected at the exposure position in the spectral layout.
In the prior art as shown in FIG. 10A, however, the diffracted light principal ray emitted from the center of the concave diffraction grating enters a photo detector surface 26 of a plane obliquely at a spectrograph forming position. Consequently, when the position of detector surface 26 is changed due to thermal deformation by temperature changes as shown in FIG. 10A, the spectral position varies on the detector surface 26 accordingly. The conventional apparatus must be calibrated intermittently with a reference light.
To prevent such a fluctuation of spectrum position, as shown in FIG. 10B, a telecentric configuration is required for entering diffracted light 27 vertically to detector surface 26. The problem can, be solved by a structure comprising a plane diffraction grating, a collimating, and a condensing optical system. However, since the diffraction grating surface is at the incident pupil position, to realize a telecentric configuration, the distance between the diffraction grating and the spectral condensing plane must be twice as long as the focal length of the condensing optical system. In the conventional apparatus, therefore, the optical path from the incident slit to the spectrograph detector surface increases, and the apparatus becomes larger.
Further, in the spectrometry measuring apparatus of a spectrographic type using array elements, a high reproducibility is required, in addition to the requirement for high resolution, so that the precision of measurement may not vary in spite of measuring environments, especially temperature changes.
A small-size and light-weight spectrometry measuring apparatus with high reliability is presented, in which the spectrum position on the detector surface does not change regardless of temperature changes during detection of such spectral intensity.
A optical system of the spectrometry measuring apparatus of the invention comprises: a condensing device operable to condense a light to be measured; a reflection type concave holographic am diffraction grating produced by exposure from two exposure light sources or a replica of the diffraction grating; and a diffracted-ray condensing device comprising a group of one or more lenses, one or more reflective mirrors having a curvature, or combined optical system of a lens and a mirror for condensing the diffracted-ray diffracted from the center of the diffraction grating telecentrically to a spectral condensing plane.
In the apparatus having such structure, since the spectral ray enters vertically to a photo detector surface disposed on a spectrum focal plane, if the photo detector is moved in the direction of an optical axis due to a temperature change, a spectrum is not moved on the photo detector surface. Consequently, it is not necessary to calibrate the optical element due to an ambient temperature change. Further, since the condensing operation of the concave holographic diffraction grating is directly utilized, it is not necessary to increase the distance between the concave diffraction grating and the spectral condensing plane, so that a small-size and light-weight spectrometry measuring apparatus may be provided.